The back-lighting in modern LCD panels (TVs, monitors) is generated by light-emitting diodes (LEDs). A large number of LEDs are required to achieve a homogeneous illumination of the entire screen. Multiple controlled current sources are used in order to deliver a load current that flows through the LEDs, which are arranged in LED chains. A corresponding arrangement can be found in FIG. 1, in which several current sources SQn are coupled to the LED chains LK. In order to reduce the power loss of the current sources SQn, the voltages VC are adjusted in such a manner that correct operation of the current sources SQn is just barely possible. For this purpose, the voltages VC are supplied to a DC/DC feedback circuit that evaluates the voltages VC and supplies the result of this evaluation to a DC/DC voltage regulator. Based on this input, the DC/DC voltage regulator transforms an input voltage VIN into an appropriate load voltage VL. The load voltage VL drops across the current source SQn and the load LK. This control loop has the effect that the current source SQn is operated at the desired operating point. Because the LED chains LK can all have a different voltage drop, all voltages VC are taken into account by the DC/DC control loop circuit. The decisive factor for the adjustment of the load voltage VL is the LED chain LK that has the highest voltage drop. The result of this is that an excessively high voltage VC is set for the remaining current sources SQn. This in turn leads to an increased power loss of the current sources in question.
The arrangement shown in FIG. 1 also has an LED monitor that checks whether an LED chain LK is interrupted or short-circuited. The detection of an interruption of an LED chain LK is necessary for such a control loop because the DC/DC feedback loop would otherwise raise the load voltage VL more and more. Because this effect leads to high power losses at the other current sources SQn, the interrupted LED chain LK must be switched off. The detection of a partial or complete short circuit in an LED chain LK is necessary in order to prevent thermal destruction of the affected current source SQn due to high power loss.
FIG. 2 shows the curves of the load voltage VL and the voltage VC for different operating situations. If an arrangement according to FIG. 1 is put into operation, the voltages VL and VC initially rise, until an optimum operating range (VCmin, VLmin) is reached. A typical load voltage runs in the range between 5 and 500 V, depending on the number of LEDs that are connected in series. Because the temperature, and therefore the voltage drop on the LED chains, varies in the course of operation, the load voltage VL also changes. FIG. 2 also shows different operating situations for the voltage VC (VC(a) to VC(e)). The difference between the voltage VL and the voltage VC designates the voltage drop on the corresponding LED chain LK. The voltage VC(a) shows an application case for dimming an LED chain LK. Dimming an LED chain LK is accomplished via a pulse-width modulation of the voltage signal VC. The pulse-width modulation switches the current source SQn on and off in defined time intervals and thus causes dimming of the LED chain LK. The load voltage VL is held constant. The voltages VC(b) and VC(c) show the voltage curve on different LED chains LK. This shows, for the sake of example, that the voltage drop at different LED chains LK can be of different magnitudes. VC(c) is the relevant voltage here for regulating the load voltage VL, because it has a lower value than the voltage VC(b). The voltage VC(d) shows an example for an LED chain that has an interruption. Therefore, the entire load voltage VL drops at the LED chain. There is no longer any voltage drop at the current source SQn. The voltage VC(e) shows a voltage curve for a partially short-circuited LED chain LK. Therefore, an increased voltage V(e) drops at the current source SQn due to the short circuit inside the LED chain LK and leads to an increased power loss.
From the descriptions for FIGS. 1 and 2, it follows that a DC/DC feedback circuit is necessary in order to reduce the voltage drop at the current sources SQn. Without this regulation, there is an excessively high loss of power at the current sources SQn. The detection of an interruption inside the LED chain LK is also necessary in order to exclude the relevant LED chain LK from the evaluation by the DC/DC feedback circuit. An interruption of the LED chain LK would otherwise lead to a falsified evaluation in the DC/DC feedback circuit and thus to an excessive load voltage VL. The recognition of a short circuit inside an LED chain LK is additionally necessary in order to protect the affected current source SQn from excessive power losses and therefore from thermal destruction. Furthermore, the current sources SQn should also be located outside of an integrated circuit in the case of load currents through the LED chains LK exceeding the value of 100 mA, because the power loss that occurs would be too high, particularly on smaller modules. This leads to an integrated control circuit that comprises the DC/DC feedback circuit and the LED monitor from FIG. 1 and external current sources SQn, which are electrically coupled to the integrated control circuit. The integrated control circuit acquires the voltages VC from the LED chains LK in order to adjust the DC/DC voltage regulator therewith and to be able to handle the LC monitoring functions. For this tapping of the voltages VC, however, the integrated monitoring circuit requires a connection that is designed even for high voltages.